Fish 101: How to do a Water Change

Whether you are a new fish-keeper or are just thinking about getting your first aquarium, maintenance can seem complex and intimidating. You may hear people talking about their equipment and their water chemistry, which can make taking care of an aquarium seem like a lot of work. But the most essential part of regular aquarium maintenance is water changes, which are quick and easy to do.

What Are Aquarium Water Changes?

Man Performing Water Change for Large Tropical Fish Tank


When we talk about water changes for aquariums, we are talking about partial water changes. You never do a full (or 100%) water change on an established aquarium. Instead, you will be taking out about a quarter of the water and replacing it with clean, dechlorinated water. 

There are a lot of reasons why we never do a complete water change, the biggest being that fish are very sensitive to changes in their water, even if the new water is an improvement over the old water. Too large of a change can shock your fish, which can make them sick or even kill them. These weekly partial water changes introduce clean, fresh water without changing the conditions in your tank too drastically.  

Equipment For An Aquarium Water Change

Siphoning gravel in a tank

You don’t need much to do a water change and tank cleaning. The most important things are:

  • Gravel vacuum – This is a siphon that pulls water out of your aquarium, and waste along with it. Be sure to get an appropriate size for your aquarium. When in doubt get a smaller one than you think you need – they are very efficient, and if you get one that’s too big, then you’ll find half the water is gone before you’ve finished cleaning the bottom of your tank! We recommend Lee’s Gravel Vacs.
  • Upgraded gravel vacuum -- If you have a larger tank (40 gallons or more), consider getting a gravel vac that hooks directly to your sink. The initial investment is a bit bigger, but they eliminate the need for you to lug heavy buckets back and forth to the sink. We recommend Lee's or Aqueon.
  • 5-gallon bucket – Your bucket should be specifically for your fish tank, and it should never have had any chemicals or cleaners in it since they can leach into the water and harm your fish. We recommend picking one up from a home improvement store.
  • Dechlorinator – There are a bunch of different options out there, and they all work well. Fish experts will all have their favorites, but anything that you choose will be fine. We use and recommend API Stress Coat and Seachem Prime
  • Tap water – Portland’s tap water is great for aquariums once it’s dechlorinated. Most municipal water systems have tap water that works well in aquariums, though each will have slightly different water chemistry. The parameters of the water can vary by season or by region, but it shouldn’t cause any problems for you. If you have a well, be sure to test your water and make sure it’s safe for aquaria before using it.
  • Net –You’ll use a net to fish out any larger pieces of debris like dead leaves or uneaten food.
  • Algae scrubber– These easily take off algae without scratching the glass or damaging your decorations. Don’t use a regular sponge – they have chemicals in them that can harm your fish. 
  • Complete test kit – When you use test kits, you really know what’s going on in your tank, AND you get to feel like a super-smart and awesome scientist while you use them. You can even buy a lab coat and an Einstein wig to impress your friends and family! We recommend API’s Freshwater Master Test Kit

How To Do An Aquarium Water Change

Water changes are very simple to do. The basic steps of changing your water are:

  1. siphon a quarter of the water out of your aquarium
  2. dispose of that water 
  3. refill your aquarium with fresh, dechlorinated water.

Water changes can be done quickly and easily, and the more you do them, the easier (and faster) they’ll be. Still worried? Here are a few more details.

Step 1: Siphon water out of your aquarium

For New Tanks Only

Brand new tanks go through something called “cycling” where the beneficial bacteria is growing to match the amount of fish (and their waste) that you have in the tank. The cycling process usually lasts for 4-6 weeks, and during the cycling process, you don’t want to change the filter or disturb the gravel in your aquarium. During this process, you will not use your gravel vacuum, so instead of siphoning out the water, you will just scoop an appropriate amount of water out of your aquarium. That’s it – this step is really that easy in new tanks, and not much more work even when your tank is fully cycled! If you’re not fully cycled yet, you can skip on to the next step. 

Not sure if you’re fully cycled? Bring in a water sample, and we’ll test it for free! Our aquatics-experts will let you know where you are in the cycling process, what to expect, and what you need to do. 

For Established Tanks

Once your tank is fully cycled, you can start using the gravel vacuum during your water changes. Put the chamber of your gravel vacuum in the water and let the hose hang down into your bucket. Make sure the hose is secure in the bucket by using a decoration or rock to brace the hose against the side of the bucket – this keeps water from spraying everywhere. 

Gravel vacuums have a mechanism that makes starting the water flow easy. Put the chamber of the gravel vac underwater and turn it horizontally to let it fill up with water. Then turn it vertical facing the bottom and gently start shaking it up and down. The valve at the top of the chamber will allow water to go up into the tubing and start the water flowing out of your tank and into the bucket. Gently press the chamber of the gravel vacuum into the gravel, and you’ll see the debris and waste get whisked out of your tank and into the bucket. The gravel will be too heavy to get sucked away and will settle back at the bottom of your tank. 

Remember – don’t remove more than 25% of the water! Doing too large of a water change can shock your fish, leading to illness or death. When you’re still getting used to your gravel vacuum, you may find that the water is siphoning out faster than you expected. If it’s going too fast for you to do the whole bottom of your tank, you can gently kink the hose part of your gravel vac, which will slow down the water flow. It’s also okay to vacuum only part of your aquarium, as long as you scoop out the larger pieces of debris and other waste with your net.  

Step 2: Dispose of the Water

Carry your bucket to the sink and pour the dirty tank-water away. Careful – the bucket may be heavy! You can also use the old tank water for your garden or your houseplants – the very things you’re trying to remove from your tank (nitrates and phosphates) are the things plants love best! Rinse out the bucket a bit to remove any residual goop, and then you’re ready to move on to the next step.

Step 3: Refill your aquarium with fresh, dechlorinated water. 

We recommend using tap water because it has trace minerals in it that fish need. Don’t use RO, bottled, or purified water unless you’re adding those trace elements back by adding buffers (powdered minerals) to the water. Most metro areas have tap water that’s great for freshwater aquariums. Portland has terrific water, though its parameters do vary a bit over the year based on the reservoir that we’re getting the water from.

Make sure that the new water’s temperature is close to the temperature in your tank. We recommend being within 5 degrees of your tank water to avoid shocking your fish. You don’t need to use a thermometer to test this – if the temperature feels the same to you, then it’s probably close enough. 

Add dechlorinator to the bucket of fresh tap water. There should be instructions on the back of the bottle. They are all easy to use – add the recommended amount to the water in the bucket, and it’ll neutralize the chlorine and chloramines in the water. Estimating is fine, and it’s better to overestimate than underestimate. But don’t double the dose unless directed to by an aquatics expert –that can cause problems in the water chemistry of your tank. 

Now add the water into the tank. Add the water slowly to avoid shocking your fish. Rearrange any decorations that were displaced and watch your fish enjoy their lovely, clean aquarium.

Saltwater tank

That’s it – you can see that water changes are very simple! While there is some equipment that you need, it’s not much, and it’s not hard to use. The whole process is easy to do and doesn’t take long. Changing your water regularly is the most important thing that you can do to keep your fish happy and healthy and looking beautiful.